The one on eBay (from China) uses CP2102, which I personally like better than FTDI chip, but there are more FTDI documentation for communication with Arduino.

Just keep in mind, eBay one ships from China and it's free shipping (meaning it's probably is going to ship via their postal service) which may take up to a few months to arrive.

If you want to get it from the US here are couple of links: CP2102 based adapter: http://www.robotsimple.com/USB_to_Serial_AdapterFTDI based adapter:http://www.robotsimple.com/Arduino_shields_and_accessories/FTDI_Friend_plus_extras_v1.0

The Chinese electronics I've gotten from Ebay were sent by airmail and arrived in 7-14 days by registered USPS. The registered mail is good in that order doesn't get lost, but bad as you have to be home to sign for it or pick it up at the post office when they are open.

Just be aware that if you use a CP2102 on a Mac the auto reset won't work if you are running Snow Leopard (earlier OS versions do work). It is the drivers. I contacted SL to tell them this over a year ago and they said the would produce a new version of the drivers but nothing yet.